Conventional techniques exist for conducting mass spectrometric analysis of large molecules using MALDI (matrix-assisted laser desorption and ionization) plates. Typically with these techniques, liquid solutions (including e.g., peptide, protein, and energy absorbing matrix) are initially introduced to pre-defined target sites on a MALDI plate. Since the diameter of the target sites are generally small and often densely packed, small (e.g., 1-5 microliter) droplets of the liquid solutions are disposed onto the plate target sites to achieve proper sample placement and to avoid sample overlap between target sites. Once disposed, the liquid samples are evaporated, with matrix crystal conglomerate containing analyte molecules (e.g., peptides and proteins) remaining on the target sites having favorable characteristics for the MALDI process and mass spectrometric analysis. Where larger conglomerate samples are desired, serial liquid sample placement and evaporation has been used to iteratively build-up a conglomerate.
Prior to placement on the MALDI plate, the liquid solution is typically purified and desalted. Such purification and desalting is achieved in the prior art with columns, pipette tips or multi-well filter plates being packed with C18 media or other chromatography media. Examples of columns packed with C18 media include Pierce PepClean™ C18 Spin Columns and 3M Empore™ Extraction Disk Cartridge. Examples of pipette tips packed with C18 media include Millipore ZipTip® Pipette Tips and Varian OMIX Pipette Tips C18. Examples of multi-well filter plates packed with C18 media include Millipore Zip Plate Micro-SPE Plate and 3M Empore™ 96-Well Extraction Disk and Plate.
For exemplary purposes, a common procedure for purifying and desalting liquid solution samples using one of the above-identified devices includes initially wetting the C18 media with a buffer containing an organic solvent, such as methanol or acetonitrile, and thereafter, equilibrating the C18 media with an equilibration buffer. The liquid solution samples are then flowed through the C18 media; it is expected that peptides and proteins of the sample be retained in the C18 media during this step. Thereafter, the C18 media is washed with a washing buffer, and, again, the peptides and proteins are expected to be retained in the media during this step. Finally, a flow elution buffer is passed through the C18 media which disassociates the peptides and proteins from the media, and it is expected that the peptides and proteins flow out with the elution buffer in this step.
A significant portion of the peptides and proteins in the sample solution may flow through the C18 media or other chromatography media in the prior art devices without binding to the C18 or other media. As a result, recovery of the peptides and proteins may be poor, particularly where the sample concentration is low.